Electron multiplier



Feb. 11, 1941. R. L. SNYDER JR ELECTRON MULTIPLIER Filed Sept. 30, 1959I)! III I /'A\ a Gttorneg Patented Feb. 11, 1941 UNITED STATES PATENTOFFICE ELECTRON MULTIPLIER Richard L. Snyder, Jr., Glassboro, N. J.,assignor to Radio Corporation of America, a corporation of Delaware Mypresent invention relates to electron discharge tubes and particularlyto improvements in electron multipliers.

The efficiency of an electron multiplier is determined to a largeextent, upon its ability to focus or otherwise direct the electrons fromone stage upon or to the next succeeding electrode. One factorinhibiting perfect focusing is the tendency of the electron stream tospread laterally when it has been augmented by more than a few stages ofelectron multiplication. This may result in the electrons spilling overthe sides of the electrodes and creating tube noises.

Another diificulty encountered in the operation of multi-stage electronmultipliers is that when the electron stream has been increased byseveral stages of electron multiplication, positive ions may begenerated in the residual gas. This phenomenon has been observed toexist even in the most thoroughly evacuated tubes. These positivelycharged particles flow in a direction opposite to the general directionof the electron stream and may impinge upon the primary-electronemitting cathode, or upon an intermediate secondary-electron emitter,where they release still more electrons. The electron current may thusbe augmented to such a degree as to render it uncontrollable. Thisbuilding up of the electron current is limited only by the so-calledspace charge effect and may result in an equilibrium of the outputcurrent, i. e., the output current may no longer accurately reflectvariations in the input current.

Rajchman and Pike, in their copending application Serial No. 171,916,filed October 30, 1937, describe an open sided electrode assembly forelectron multipliers wherein each emitter electrode has a surface whichextends. into the space between the cathode and anode whereby theseseveral surfaces constitute a barrier to the passage of ions in theanode-cathode directions. Such construction bars or entraps anddissipates the vast majority of ions adjacent their area of origin andthereby ensures improved performance. The improved performance is bestevidenced by the fact that theoutput current is directly proportional tothe input current substantially irrespective of the intensity of theelectron stream impinging upon the output electrode. It has beenobserved, however, that when the Raj chman and Pike tubes are operatedat or close to their maximum output, there is a tendency for the tube tobecome noisy.

I have discovered that this undesired result may be attributed to theescape of some ions from the ion traps due to the distorted contour ofthe equipotential lines which may be present adjacent the lateral edgesof the open sided electrode assembly due to the proximity of the glasswalls of the envelope.

As a result of numerous observations, I con-- clude that these escapedions flow to the low current (input) end of the device, where theyapparently impinge upon the inner surface of the container, thuscreating a positive charge thereon. When the positive charge on the tubewall extends to the area which surrounds the cathode, the positivesurface will draw some of the emitted primary or photo-electronsthereto. These negatively charged particles neutralize the positivecharge on the glass wall and in so doing create an electricaldisturbance Within the tube. Such disturbance will ordinarily be of acontinuing nature because the positive charge having been neutralized,the escaping ions will again be drawn thereto, followed later by theelectrons from the cathode which provide the disturbance incident to thedescribed neutralizing action.

Accordingly, the principal object of my pres ent invention is to providean electron multiplier wherein tube noises due to the flow of ions or tolack of perfect focusing of the electrons is substantially obviated.

As described in my copending application Serial No.'177,286 filedNovember 30, 1937, I have heretofore achieved the abovementioned generalobject in an electron multiplier of the type described in the saidRajchman and Pike application by supporting the electrode assembly on aplurality of spaced discs which extend to the tube wall and act asphysical barriers to the passage of escaped ions. These discs arepreferably each provided with a pair of baffle which span the open sidesof the electrode assembly and serve to extend the equipotential surfacesbetween adjacent elec trodes out and beyond the edges of said electrodeswhereby the electrically charged particles generated within the assemblyare constrained to remain within the desired boundaries instead of beingdrawn outwardly by the distorted field usually present adjacent the sideedges of the multiplying electrodes.

I have now discovered that the same desirable results can be achieved ina more economical and facile manner (and without the use of auxiliarydiscs and bafiles) by so shaping the electrodes that the electrons arefocused inwardly toward the center of the next electrode. This permitsthe use of an extremely compact enclosed electrode assembly from whichthe ions cannot escape -to be shined upon the cathode.

and one which, by reason of its simplicity of construction and ease ofassembly, lends itself readily to mass production methods.

Other objects and advantages, together with certain details ofconstruction, will be apparent and my invention itself will be bestunderstood by reference to the following specification and to theaccompanying drawing, wherein Figure 1 is a sectional view on anenlarged scale taken on the line I--l of Fig. 3 and showing an electronmultiplier embodying my invention:

Figure 2 is a view in perspective of one of the multiplying electrodesof the device of Fig. 1,; and

Figure 3 is a side elevational view, partly in section, of the electronmultiplier of Fig. 1.

The general arrangement of the electrodes may be similar to that claimedin copending application Serial No. 187,634 to Zworykin and Snyder,filed January 29, 1938. In the drawing, wherein like referencecharacters designate the same parts in all figures, T designatesgenerally a highly evacuated cylindrical envelope or tube, thelongitudinal axis of which is marked X. The tube has a pair of opposed,preferably flat, inner surfaces A and B, respectively, (see Fig. 3) andis provided with an inner radial partition C which extends from a pointadjacent the central axis X to the inner wall thereof and serves toprevent the passage of electrons from the low voltage to the highvoltage side of the tube. Considered from another aspect, the partitionC serves to prevent the passage of any ions, which may escape the laterdescribed ion barriers, from being drawn to the low voltage side of thedevice. Perhaps more important is the fact that this partition Cprevents cold discharge of electrons which might otherwise be caused bythe high electric field which would otherwise exist between the high andlow voltage sides of the dev1ce.

Where, as in the drawing, the device is provided with a photosensitivecathode P, the envelope is preferably constituted, in whole or in part,of glass, Pyrex, or other suitable transparent material to permit light(exemplified by the line a) from an external source, not shown, A gridG, which is preferably connected to the photo-sensitive cathode P and issupported between it and the inner wall of the glass envelope T, servesto prevent the accumulation of a positive charge upon the inner surfaceof the envelope. In addition to the grid G and cathode P, the tubecontains a plurality of intermediate electron multiplying electrodes,designated l to 8, inclusive, which are mounted in staggered relation onopposite sides of a curved substantially arcuate median line m, whichmay be saidto bisect the electron path which extends from the cathode Pto and beyond the last mentioned multiplying electrode 8. Anotherelectrode 9, which may extend to or across the median line m, comprisesthe last multiplying stage of the device. As described in copendingapplication Serial No. 287,020, filed July 28, 1939, to Rajchman andSnyder, this final multiplying electrode 9 preferably comprises aC-shaped or inflated U- shaper plate having the open spacebetween itsinwardly directed terminal edges directed toward the source 8. Thecollector electrode or anode H3 is mounted within the oval defined bythe inner surface of the last multiplying electrode 9. The collectoranode I0 may be of gridlike or other openwork construction to permit theelectrons from the source 8 to, pass there through and to impinge thesecondarily emissive surface of electrode 9 without substantialnumerical diminution.

As indicated by the arrows on the dotted lines extending between theseveral electrodes, the electrons leaving the photosensitive cathodeadvance in the anode direction in travelling to the next higher numberedelectrode by reason of the electrostatic fields which are generatedtherebetween when each is maintained at a fixed potential which ishigher than that of its preceding (in point of electron travel)electrode.

It will be observed from an inspection of Fig. 1 that the emissivesurface e of each of the multiplying electrodes I to 8 inclusive isnon-cylindrical, or cup-shaped, or concave spherical construction, i. c.it is curved not only in the direction of the median line 122 (as in theprior art) but is also curved, in accordance with my present invention,in the transverse direction. The two directions of curvature areindicated by the crossed lines on the back of the emissive surface ofthe multiplying electrode shown in Fig. 2. This double curvature of themultiplying surfaces 6 provides an electrostatic field which directs theelectrons inwardly toward the center of the next succeeding electrode.

It will also be observed from an inspection of Fig. 1 that the spacebetween each multiplying electrode of both the inner (even numbered) andthe outer (odd numbered) sets of electrodes is closed by an insulatingrod R. which serves as a support for two adjacent electrodes. Atransverse linear non-emissive portion of each multiplying electrodeadjacent each leading f (nearer the anode) and trailing g (nearer theoathode) edge is bent to fit snugly about the rods R without contactingthe next electrode of the same set. Referring to Fig. 3, it will be seenthat each multiplying electrode spans the entire distance between theopposed fiat inner surfaces A and B of the tube or envelope T. Sincethese surfaces Aand B close the sides of the electrodes and the rods Rclose the space between adjacent electrodes of the same set, theelectrode assembly considered in its entirety may be said to comprise aconduit through which the electrons may travel from one electrode toanother.

As shown more clearly in Fig. 2, the ends of the electrodes which lieadjacent the flat tube surfaces are preferably provided each with arearwardly bent flange h which lends rigidity to the assembly. In orderto reduce dark current (i. e. current which flows when the cathode isnot illuminated but the electrodes are energized), it is preferable toround all of the edges of the electrodes along the areas wherefieldemission might arise.

Another and important feature of my present invention resides inproviding the trailing end of the multiplying electrodes with apreferably rounded extension 7' which projects beyond the leading edgeof the next preceding multiplying electrode of the same (inner or outer)set. When the electrodes are energized these extensions 7' may be saidto introduce an accelerating component into the electrostatic fieldbetween each opposite pair of electrodes without detrimentally affectingthe focusing of the electron stream and without either increasing thenumber of electrodes or complicating the electrode assembly.

In manufacturing the electron multiplier in the form shown in Fig. 3,the glass envelope is made in two sections ti and t2. The stem part tlof the tube contains the fiat inner base to which the insulatingsupporting rods R are affixed. The rods B may be of glass and madeintegral with the base A. The electrodes may thus be mounted in placesimply by sliding their complementary parts I and 9 about the rods 9.The upper or cap portion 152 is then fitted over the stem portion ti andsealed thereto, as indicated at s, and the tube pumped in usual mannerthrough a tip, not shown, on its lower rim. Since it will be understoodthat the leads for the several electrodes may extend through the base A,it would needlessly complicate the drawing to show them.

While my invention has been described as applied to an electronmultiplier wherein the electrodes are mounted on opposite sides of anarcuate median line which extends between the cathode and anode, it willbe apparent that my invention is none the less applicable to tubeswherein the electrodes are arranged along a straight axis. It is tolbeunderstood, therefore, that the foregoing is to be interpreted asillustrative and not in a limiting sense, except as required by thespirit of the appended claims.

The term non-cylindrical surface as herein used is to be interpreted inits accepted geometrical sense. It is thus not intended to include asurface which lies in a single plane.

What is claimed is:

1. An electron multiplier comprising an evacuated envelope having a pairof opposed substantially fiat inner walls, a cathode, an anode and apair of sets of spaced multiplying electrodes extending between saidflat walls and mounted in staggered relation on opposite sides of amedian line which extends between said cathode and anode, and a.plurality of insulating members supporting the multiplying electrodes ofeach set, said insulating members extending between said fiat walls andclosing the spaces between the spaced electrodes of the same set.

2. An electron multiplier comprising an evacuated envelope having a pairof opposed substantially fiat inner surfaces, a pair of rows ofinsulating supporting rods extending between said surfaces, and a pairof sets of spaced multiplying electrodes mounted on said supporting rodsand spanning the space between said fiat surfaces, said supporting rodsclosing the spaces between the electrodes which they support.

3. An electron multiplier comprising an evacuated envelope having a pairof opposed inner surfaces, a pair of rows of insulating supporting rodsextending between said surfaces, a pair of sets of spaced multiplyingelectrodes mounted on said supporting rods, the electrodes of each setbeing spaced from each other to form a conduit through which electronsmay travel from one electrode to another, the side walls of said conduitcomprising the said opposed surfaces ofsaid envelope and the spacesbetween the electrodes of each set being closed by said supporting rods.

4. The invention as set forth in claim 3 wherein one end of said conduitis closed by a multiplying electrode which spans the space between saidsets of multiplying electrodes.

5. An electron multiplier comprising a cathode, an anode, and a set ofmultiplying surfaces mounted in staggered relation on each side of amedian line which extends between said cathode vand anode, the trailingtransverse edges of the multiplying electrodes of each set being bentinwardly toward said median line and projecting beyond the leading edgeof the next preceding electrode in the same set.

RICHARD L. SNYDER. JR.

